Semiconductor device

ABSTRACT

An object of the invention is to improve the accuracy of light detection in a photosensor, and to increase the light-receiving area of the photosensor. The photosensor includes: a light-receiving element which converts light into an electric signal; a first transistor which transfers the electric signal; and a second transistor which amplifies the electric signal. The light-receiving element includes a silicon semiconductor, and the first transistor includes an oxide semiconductor. The light-receiving element is a lateral-junction photodiode, and an n-region or a p-region included in the light-receiving element overlaps with the first transistor.

TECHNICAL FIELD

The technical field relates to a photosensor, a semiconductor deviceincluding the photosensor, and a driving method thereof.

BACKGROUND ART

In recent years, attention has been driven to semiconductor devicesprovided with light-detecting sensors (also referred to as photosensors)(see Patent Document 1).

Semiconductor devices provided with photosensors include CCD imagesensors, CMOS image sensors, and the like. Such image sensors are used,for example, in electronic apparatuses like digital still cameras orcellular phones. Further, as semiconductor devices includingphotosensors in their display portions, touch panels and the like havebeen developed.

In a semiconductor device including a photosensor, light emitted from anobject to be detected or external light reflected by the object to bedetected is detected directly by the photosensor or condensed by anoptical lens or the like and then detected.

REFERENCE Patent Document

[Patent Document 1] Japanese Published Patent Application No.2001-292276

DISCLOSURE OF INVENTION

An object of the present invention is to improve the accuracy of lightdetection in a photosensor.

It is another object to provide a new layout or structure of a circuitof a photosensor. In particular, an object of the present invention isto increase a light-receiving area.

One embodiment of the present invention is a semiconductor deviceincluding: a light-receiving element which converts light into anelectric signal; a first transistor which transfers the electric signal;and a second transistor which amplifies the electric signal. Thelight-receiving element includes a silicon semiconductor, and the firsttransistor includes an oxide semiconductor.

The light-receiving element is a lateral-junction photodiode, and ann-region or a p-region included in the light-receiving element overlapswith the first transistor.

The light-receiving element is formed over the same surface as thesecond transistor.

A wiring formed over a light-receiving region of the light-receivingelement is made of a light-transmitting material.

In this specification, the semiconductor device refers to an elementhaving a semiconductor property, and all the object including theelement. For example, a display device including a transistor is simplyreferred to as a semiconductor device in some cases.

In the photosensor, an oxide semiconductor is used for the firsttransistor which transfers an electric signal, resulting in a reductionin the leakage current of the first transistor in the off state and animprovement in the accuracy of detecting light.

Furthermore, since the n-region or the p-region included in thelight-receiving element overlaps with the first transistor, thelight-receiving area of the light-receiving element can be increased;accordingly, high light sensitivity can be realized and light can bedetected with high accuracy.

In addition, since the wiring formed over the light-receiving region ismade of a light-transmitting material, the light-receiving area can beincreased; accordingly, high light sensitivity can be realized and lightcan be detected with high accuracy.

BRIEF DESCRIPTION OF DRAWINGS

In the accompanying drawings:

FIG. 1 illustrates an example of a circuit of a semiconductor device;

FIG. 2 illustrates an example of a layout of a semiconductor device;

FIG. 3 illustrates an example of a layout of a semiconductor device;

FIG. 4 illustrates an example of a layout of a semiconductor device;

FIG. 5 illustrates an example of a layout of a semiconductor device;

FIG. 6 illustrates an example of a layout of a semiconductor device;

FIG. 7 illustrates an example of a circuit of a semiconductor device;

FIG. 8 illustrates an example of a layout of a semiconductor device;

FIG. 9 illustrates an example of a layout of a semiconductor device;

FIG. 10 illustrates an example of a layout of a semiconductor device;

FIG. 11 illustrates an example of a layout of a semiconductor device;

FIG. 12 illustrates an example of a layout of a semiconductor device;

FIG. 13 illustrates an example of a circuit of a semiconductor device;

FIG. 14 illustrates an example of a layout of a semiconductor device;

FIG. 15 illustrates an example of a layout of a semiconductor device;

FIG. 16 illustrates an example of a layout of a semiconductor device;

FIG. 17 illustrates an example of a layout of a semiconductor device;

FIG. 18 illustrates an example of a layout of a semiconductor device;

FIG. 19 illustrates an example of a cross-sectional structure of asemiconductor device;

FIG. 20 illustrates an example of a cross-sectional structure of asemiconductor device;

FIG. 21 illustrates an example of a circuit of a semiconductor device;

FIG. 22 illustrates an example of a layout of a semiconductor device;

FIG. 23 illustrates an example of a cross-sectional structure of asemiconductor device;

FIG. 24 illustrates an example of a cross-sectional structure of asemiconductor device;

FIG. 25 illustrates an example of a semiconductor device;

FIG. 26 illustrates an example of a semiconductor device;

FIGS. 27A to 27F illustrate examples of a circuit of a semiconductordevice;

FIG. 28 is a graph showing characteristics of a semiconductor device;and

FIG. 29 is a timing chart.

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiments will be described in detail below with reference todrawings. Note that the following embodiments can be implemented in manydifferent modes, and it is apparent to those skilled in the art thatmodes and details can be modified in various ways without departing fromthe spirit and scope of the present invention. Therefore, the presentinvention is not construed as being limited to the description of theembodiments. Note that in all the drawings for explaining theembodiments, like portions or portions having a similar function aredenoted by like reference numerals, and the description thereof isomitted.

(Embodiment 1)

In this embodiment, the circuit and layout of a semiconductor devicewill be described.

FIG. 1 is an example of a circuit diagram of a photosensor.

The photosensor includes a photodiode 100, a transistor 101, atransistor 102, a transistor 103, and a transistor 104.

The photodiode 100 has a function of converting light into an electricsignal (an electric charge). Other than the photodiode, alight-receiving element having this function, such as a phototransistor,can also be used.

The transistor 101 has a function of transferring the converted electricsignal to a gate of the transistor 103. Thus, the transistor 101 is alsoreferred to as a transferring transistor.

The transistor 102 has a function of controlling a gate potential of thetransistor 103 so that the gate potential is reset to a predeterminedpotential. Thus, the transistor 102 is also referred to as a resettransistor.

The transistor 103 has a function of amplifying the transferred electricsignal and generating an output signal. Thus, the transistor 103 is alsoreferred to as an amplifying transistor. Note that the amplificationmeans here that a current value between a source and a drain iscontrolled with a gate potential.

The transistor 104 has a function of controlling the reading of theoutput signal. For example, among a plurality of photosensors, theoutput from a predetermined photosensor is selected by the transistor104; thus, the transistor 104 is also referred to as a selectiontransistor.

In the circuit of FIG. 1, a gate of the transistor 101 is electricallyconnected to a wiring 106 (also referred to as a charge control signalline), one of a source and a drain of the transistor 101 is electricallyconnected to one electrode of the photodiode 100, and the other of thesource and the drain of the transistor 101 is electrically connected toone of a source and a drain of the transistor 102 and the gate of thetransistor 103. A gate of the transistor 102 is electrically connectedto a wiring 107 (also referred to as a reset signal line), and the otherof the source and the drain of the transistor 102 is electricallyconnected to a wiring 108 (also referred to as a power supply line). Oneof a source and a drain of the transistor 103 is electrically connectedto the wiring 108, and the other thereof is electrically connected toone of a source and a drain of the transistor 104. A gate of thetransistor 104 is electrically connected to a wiring 109 (also referredto as a selection signal line), and the other of the source and thedrain of the transistor 104 is electrically connected to a wiring 110(also referred to as an output line). Note that the other electrode ofthe photodiode 100 is electrically connected to a wiring 120. Thepotential of the wiring 120 can be set to a desired potential: either afixed potential (e.g., the ground potential) or a variable potential.

FIG. 2 is an example of the layout of the photosensor.

The photosensor includes a light-receiving element (the photodiode 100)and the four transistors 101 to 104. The photodiode 100 is alateral-junction PIN photodiode, in which an n-region 201, an i-region202, and a p-region 203 are formed over the same surface. The transistor101 and the transistor 102 are formed over the n-region 201. Thelight-receiving area can be increased with the layout in which part orthe whole of the transistor 101 overlaps with the n-region 201 that isnot a light-receiving region.

Note that the transistor 101 may be formed over the p-region 203 that isnot a light-receiving region. The photodiode 100 can be a PN photodiodewithout an i-region.

In this embodiment, an oxide semiconductor is preferably used for thetransistor 101 and the transistor 102. Such a structure allows reducingthe leakage of the electric signal, which has been supplied to the gateof the transistor 103, from the transistor 101 and the transistor 102.This is because a transistor using an oxide semiconductor has a lowleakage current in the off state. Consequently, light can be detectedwith high accuracy. The structure is particularly effective in the casewhere it takes a long time between light reception and reading out.

The structure is also effective in a semiconductor device including aplurality of photosensors (e.g., FIG. 25 or FIG. 26), in the case wherethe time between light reception and reading out varies from photosensorto photosensor. There are structures where light is simultaneouslyreceived in all the photosensors and reading is sequentially performedfor each line.

Note that the photodiode 100, the transistor 103, and the transistor 104are formed by making use of the same semiconductor material. Since thephotodiode 100, the transistor 103, and the transistor 104 can be formedover the same surface in the same process, cost reduction can beachieved. When a semiconductor with high mobility is used, the quantumefficiency of the photodiode 100 can be increased, and amplification bythe transistor 103 and reading by the transistor 104 can be performedefficiently. Here, a crystalline semiconductor is used. In particular,single crystal silicon is preferably used, though other semiconductors,such as an amorphous semiconductor or an oxide semiconductor, can beused as needed.

FIG. 3 to FIG. 6 are other examples of the layout of the photosensor,each of which is different from FIG. 2 in the light-receiving area ofthe photodiode 100.

Note that in FIG. 3, FIG. 5 and FIG. 6, a light-transmitting material isused for a wiring (a conductive layer 130) that overlaps with alight-receiving region of the photodiode 100 (here, the i-region 202).Light passes through the conductive layer 130 and enters the i-region202; accordingly, the light-receiving area can be increased.

As the light-transmitting material, it is possible to use, for example,indium tin oxide (ITO), indium tin oxide containing silicon oxide(ITSO), organoindium, organotin, zinc oxide, indium zinc oxide (IZO),zinc oxide containing gallium, tin oxide, indium oxide containingtungsten oxide, indium zinc oxide containing tungsten oxide, indiumoxide containing titanium oxide, or indium tin oxide containing titaniumoxide.

Furthermore, in FIG. 3, the transistor 101 and the transistor 102 areformed to overlap with the i-region 202 of the photodiode. Thisstructure makes use of the transmitting property of the oxidesemiconductor: light passes through the transistor 101, the transistor102, and the conductive layer 130 and enters the i-region 202.Accordingly, the light-receiving area can be increased.

When the light-receiving area is increased in the aforementioned manner,light sensitivity can be increased and thus light can be detected withhigh accuracy.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 2)

This embodiment shows an example of the circuit and layout of thesemiconductor device, which is different from the example shown inEmbodiment 1.

FIG. 7 is an example of the circuit diagram of the photosensor, in whichthe transistor 102 of FIG. 1 is omitted.

FIG. 8 to FIG. 12 are examples of the layout of the circuit in FIG. 7,in which the transistor 102 of FIG. 2 to FIG. 6 is omitted,respectively. The light-receiving area can be increased because of areduction in the number of elements.

FIG. 13 is an example of the circuit of the photosensor, in which thetransistor 102 and the transistor 104 of FIG. 1 are omitted.

FIG. 14 to FIG. 18 are examples of the layout of the circuit in FIG. 13,in which the transistor 102 and the transistor 104 of FIG. 2 to FIG. 6are omitted, respectively. The light-receiving area can be furtherincreased because of a reduction in the number of elements.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 3)

In this embodiment, a cross-sectional structure of the semiconductordevice will be described.

FIG. 19 is a cross-sectional view of the photosensor illustrated in FIG.1 to FIG. 18.

In FIG. 19, a photodiode 1002, a transistor 1003, and a transistor 1004are provided over a substrate 1001 having an insulating surface. Thephotodiode 1002, the transistor 1003, and the transistor 1004respectively illustrate an example of the cross-sectional structure ofthe photodiode 100, the transistor 103, and the transistor 101illustrated in FIG. 1 to FIG. 18.

Light 1202 emitted from an object to be detected 1201, light 1202reflected by the object to be detected 1201 (such as external light), orlight 1202 emitted from the inside of the device and reflected by theobject to be detected 1201 enters the photodiode 1002. An object to bedetected may be provided on the substrate 1001 side so that an imagethereof is taken.

The substrate 1001 can be an insulating substrate (e.g., a glasssubstrate or a plastic substrate), the insulating substrate on which aninsulating film (e.g., a silicon oxide film or a silicon nitride film)is formed, a semiconductor substrate (e.g., a silicon substrate) onwhich the insulating film is formed, or a metal substrate (e.g., analuminum substrate) on which the insulating film is formed.

The photodiode 1002 is a lateral-junction PIN photodiode and includes asemiconductor film 1005. The semiconductor film 1005 includes a regionhaving p-type conductivity (a p-region 1021), a region having i-typeconductivity (i-region 1022), and a region having n-type conductivity(n-region 1023). Note that the photodiode 1002 may be a PN photodiode.

The lateral-junction PIN or PN photodiode can be formed by adding ap-type impurity and an n-type impurity to predetermined regions of thesemiconductor film 1005.

In the photodiode 1002, a single crystal semiconductor (e.g., singlecrystal silicon) with few crystal defects is preferably used for thesemiconductor film 1005 so as to improve the proportion of an electricsignal generated from incident light (the quantum efficiency).

The transistor 1003 is a top-gate thin film transistor and includes asemiconductor film 1006, a gate insulating film 1007, and a gateelectrode 1008.

The transistor 1003 has a function of converting an electric signalsupplied from the photodiode 1002 into an output signal. Therefore, asingle crystal semiconductor (e.g., single crystal silicon) ispreferably used for the semiconductor film 1006 to obtain a transistorwith high mobility.

An example of forming the semiconductor film 1005 and the semiconductorfilm 1006 with the use of a single crystal semiconductor will bedescribed. A damaged region is formed at a desired depth of a singlecrystal semiconductor substrate (e.g., a single crystal siliconsubstrate) by ion irradiation or the like. The single crystalsemiconductor substrate and the substrate 1001 are bonded to each otherwith an insulating film interposed therebetween; then, the singlecrystal semiconductor substrate is split along the damaged region,whereby a semiconductor film is formed over the substrate 1001. Thesemiconductor film is processed (patterned) into a desired shape byetching or the like, so that the semiconductor film 1005 and thesemiconductor film 1006 are formed. Since the semiconductor film 1005and the semiconductor film 1006 can be formed in the same process, costreduction can be realized. In this manner, the photodiode 1002 and thetransistor 1003 can be formed on the same surface.

Note that an amorphous semiconductor, a microcrystal semiconductor, apolycrystalline semiconductor, an oxide semiconductor, or the like canalso be used for the semiconductor film 1005 and the semiconductor film1006. In particular, a single crystal semiconductor is preferably usedto obtain a transistor with high mobility. As the semiconductormaterial, it is preferable to use a silicon semiconductor such assilicon or silicon germanium, the crystallinity of which can be easilyincreased.

Here, the semiconductor film 1005 is preferably made thick in order toimprove the quantum efficiency of the photodiode 1002. Further, thesemiconductor film 1006 is preferably made thin in order to improve theelectrical properties such as the S value of the transistor 1003. Inthat case, the semiconductor film 1005 is only required to be madethicker than the semiconductor film 1006.

A crystal semiconductor is also preferably used for the transistor 104in FIG. 2 to FIG. 12 so as to obtain a transistor with high mobility. Byusing the same semiconductor material as the transistor 1003, thetransistor 104 can be formed in the same process as the transistor 1003,resulting in cost reduction.

Note that the gate insulating film 1007 is formed as a single layer orstacked layers using a silicon oxide film, a silicon nitride film, orthe like. The gate insulating film 1007 can be formed by plasma CVD orsputtering.

Note that the gate electrode 1008 is formed as a single layer or stackedlayers using a metal material such as molybdenum, titanium, chromium,tantalum, tungsten, aluminum, copper, neodymium, or scandium, or analloy material including any of these materials as a main component. Thegate electrode 1008 can be formed by sputtering or vacuum evaporation.

The transistor 1003 can be a bottom-gate transistor, and can have achannel-stop structure or a channel-etched structure.

The transistor 1004 is a bottom-gate inverted-staggered thin filmtransistor and includes a gate electrode 1010, a gate insulating film1011, a semiconductor film 1012, an electrode 1013, and an electrode1014. An insulating film 1015 is provided over the transistor 1004.

A feature of the structure is that the transistor 1004 is formed overthe photodiode 1002 and the transistor 1003 with an insulating film 1009interposed therebetween. When the transistor 1004 and the photodiode1002 are formed on different layers in this manner, the area of thephotodiode 1002 can be increased.

Furthermore, part or the whole of the transistor 1004 is preferablyformed to overlap with either the n-region 1023 or the p-region 1021 ofthe photodiode 1002, whereby the light-receiving area of the photodiode1002 can be increased. Also in the case of a PN photodiode, part or thewhole of the transistor 1004 is preferably formed to overlap with eitherthe n-region or the p-region.

The function of the transistor 1004 is to accumulate an electric signalsupplied from the photodiode 1002 in the gate of the transistor 1003 andto retain the electric signal. Therefore, an oxide semiconductor ispreferably used for the semiconductor film 1012 so that the transistor1004 has an extremely low off-current.

It is also preferable to use an oxide semiconductor for the transistor102 in FIG. 1 to FIG. 6 so that the transistor 102 has a lowoff-current. By using the same semiconductor material as the transistor1004, the transistor 102 can be formed over the same surface and in thesame process as the transistor 1004, resulting in cost reduction. Whenthe transistor 1004 is formed to overlap with the n-region 1023 or thep-region 1021, an increase in light-receiving area can be achieved.

An example of forming the semiconductor film 1012 using an oxidesemiconductor will be described below.

One of the factors that increase the off-current of a transistor is animpurity such as hydrogen (e.g., hydrogen, water, or a hydroxyl group)contained in an oxide semiconductor. Hydrogen or the like might be acarrier supplier (donor) in an oxide semiconductor, which causeselectric current even in the off state. That is, an oxide semiconductorcontaining a large amount of hydrogen or the like becomes an n-typeoxide semiconductor.

Thus, in the manufacturing method shown below, the amount of hydrogen inan oxide semiconductor is reduced as much as possible and theconcentration of oxygen which is a constituent element is increased,whereby the oxide semiconductor is highly purified. The highly-purifiedoxide semiconductor is an intrinsic or substantially intrinsicsemiconductor, resulting in a reduction in off-current.

First, an oxide semiconductor film is formed over the insulating film1009 by sputtering.

As a target used for forming the oxide semiconductor film, a target of ametal oxide containing zinc oxide as a main component can be used. Forexample, it is possible to use a target with a composition ratio ofIn₂O₃:Ga₂O₃:ZnO=1:1:1, that is, In:Ga:Zn=1:1:0.5. It is also possible touse a target with a composition ratio of In:Ga:Zn=1:1:1 or a compositionratio of In:Ga:Zn=1:1:2. Further, a target which includes SiO₂ at 2 wt %to 10 wt % inclusive can be used.

Note that the oxide semiconductor film can be formed in a rare gas(typically, argon) atmosphere, an oxygen atmosphere, or a mixedatmosphere of a rare gas and oxygen. Here, a sputtering gas used forforming the oxide semiconductor film is a high-purity gas in whichimpurities such as hydrogen, water, hydroxyl groups, or hydride arereduced down to concentrations of the order of ppm or ppb levels.

The oxide semiconductor film is formed by introducing a sputtering gasfrom which hydrogen and moisture are removed, while removing moistureremaining in a treatment chamber. In order to remove moisture remainingin the treatment chamber, an entrapment vacuum pump is preferably used.For example, a cryopump, an ion pump, or a titanium sublimation pump ispreferably used.

The thickness of the oxide semiconductor film can be 2 nm to 200 nminclusive, preferably 5 nm to 30 nm inclusive. Then, the oxidesemiconductor film is processed (patterned) into a desired shape byetching or the like, whereby the semiconductor film 1012 is formed.

Although an In—Ga—Zn—O is used for the oxide semiconductor film in theabove example, the following oxide semiconductors can also be used:In—Sn—Ga—Zn—O, In—Sn—Zn—O, In—Al—Zn—O, Sn—Ga—Zn—O, Al—Ga—Zn—O,Sn—Al—Zn—O, In—Zn—O, Sn—Zn—O, Al—Zn—O, Zn—Mg—O, Sn—Mg—O, In—Mg—O, In—O,Sn—O, Zn—O, and the like. The oxide semiconductor film may contain Si.Further, the oxide semiconductor film may be amorphous or crystalline.Further, the oxide semiconductor film may be non-single-crystal orsingle crystal.

As the oxide semiconductor film, a thin film represented by InMO₃(ZnO)_(m) (m>0) can also be used. Here, M denotes one or more of metalelements selected from Ga, Al, Mn, and Co. For example, M can be Ga, Gaand Al, Ga and Mn, or Ga and Co.

Next, first heat treatment is performed on the oxide semiconductor film(the semiconductor film 1012). The temperature of the first heattreatment is higher than or equal to 400° C. and lower than or equal to750° C., preferably higher than or equal to 400° C. and lower than thestrain point of the substrate.

Through the first heat treatment, hydrogen, water, hydroxyl groups, andthe like can be removed from the oxide semiconductor film (thesemiconductor film 1012) (dehydrogenation treatment). Thedehydrogenation treatment through the first heat treatment issignificantly effective because such impurities become a donor in theoxide semiconductor film and increase the off-current of the transistor.

Note that the first heat treatment can be performed in an electricfurnace. Alternatively, heat conduction or heat radiation from a heatingelement such as a resistance heating element can be used for the firstheat treatment. In that case, an RTA (rapid thermal anneal) apparatussuch as a GRTA (gas rapid thermal anneal) apparatus or an LRTA (lamprapid thermal anneal) apparatus can be used.

An LRTA apparatus is an apparatus for heating an object to be processedby radiation of light (an electromagnetic wave) emitted from a lamp suchas a halogen lamp, a metal halide lamp, a xenon arc lamp, a carbon arclamp, a high-pressure sodium lamp, or a high-pressure mercury lamp.

A GRTA apparatus is an apparatus for heat treatment using ahigh-temperature gas. As the gas, an inert gas (typically, a rare gassuch as argon) or a nitrogen gas can be used. The use of the GRTAapparatus is particularly effective because high-temperature heattreatment in a short time is enabled.

The first heat treatment may be performed before the patterning of theoxide semiconductor film, after the formation of the electrode 1013 andthe electrode 1014, or after the formation of the insulating film 1015.However, the first heat treatment is preferably performed before theformation of the electrode 1013 and the electrode 1014 so that theelectrodes are not damaged by the first heat treatment.

During the first heat treatment, oxygen deficiencies might be generatedin the oxide semiconductor. Therefore, after the first heat treatment,oxygen is preferably introduced to the oxide semiconductor (treatmentfor supplying oxygen) so that oxygen which is a constituent element ishighly purified.

Specifically, as an example the treatment for supplying oxygen, thefirst heat treatment is followed by second heat treatment in an oxygenatmosphere or an atmosphere containing nitrogen or oxygen (for example,the volume ratio of nitrogen to oxygen is 4 to 1). Alternatively, plasmatreatment may be performed in an oxygen atmosphere, whereby the oxygenconcentration in the oxide semiconductor film can be increased and theoxide semiconductor film can be highly purified. The temperature of thesecond heat treatment is higher than or equal to 200° C. and lower thanor equal to 400° C., preferably higher than or equal to 250° C. andlower than or equal to 350° C.

As another example of the treatment for supplying oxygen, an oxideinsulating film (the insulating film 1015) such as a silicon oxide filmis formed on and in contact with the semiconductor film 1012, and then athird heat treatment is performed. Oxygen in the insulating film 1015moves to the semiconductor film 1012 to increase the oxygenconcentration in the oxide semiconductor, whereby the oxidesemiconductor film can be highly purified. The temperature of the thirdheat treatment is higher than or equal to 200° C. and lower than orequal to 400° C., preferably higher than or equal to 250° C. and lowerthan or equal to 350° C. Note that also in the case of a top-gatetransistor, the oxide semiconductor can be highly purified in such amanner that a gate insulating film on and in contact with thesemiconductor film 1012 is formed of a silicon oxide film or the likeand similar heat treatment is performed.

As described above, the oxide semiconductor film can be highly purifiedthrough the treatment for supplying oxygen such as the second heattreatment or the third heat treatment after the dehydrogenationtreatment by the first heat treatment. When being highly purified, theoxide semiconductor can be made intrinsic or substantially intrinsic,resulting in a reduction in the off-current of the transistor 1004.

Note that the insulating film 1009 is a single layer or stacked layersusing a silicon oxide film, a silicon nitride film, or the like, and isformed over the photodiode 1002 and the transistor 1003. The insulatingfilm 1009 can be formed by plasma CVD or sputtering. The insulating film1009 may also be formed of a resin film such as a polyimide film bycoating or the like.

The gate electrode 1010 is formed as a single layer or stacked layersusing a metal material such as molybdenum, titanium, chromium, tantalum,tungsten, aluminum, copper, neodymium, or scandium, or an alloy materialincluding any of these materials as a main component. The gate electrode1010 can be formed by sputtering or vacuum evaporation.

The gate insulating film 1011 is formed as a single layer or stackedlayers using a silicon oxide film, a silicon nitride film, or the like.The gate insulating film 1011 may be formed by plasma CVD or sputtering.

The electrode 1013 and the electrode 1014, which are formed over thegate insulating film 1011 and the semiconductor film 1012, each are asingle layer or stacked layers using a metal such as molybdenum,titanium, chromium, tantalum, tungsten, aluminum, copper, or yttrium, analloy material including any of these materials as a main component, ora metal oxide having conductivity such as indium oxide. The electrode1013 and the electrode 1014 can be formed by sputtering or vacuumevaporation. Here, it is preferable that the electrode 1013 beelectrically connected to the n-region 1023 of the photodiode 1002through a contact hole formed in the gate insulating film 1007, theinsulating film 1009, and the gate insulating film 1011. It is alsopreferable that the electrode 1013 and the electrode 1014 be formed tooverlap with the gate electrode 1010, whereby the current drivecapability of the transistor 1004 can be increased. Such a structure isparticularly effective in the case of using an intrinsic orsubstantially intrinsic oxide semiconductor.

The highly-purified oxide semiconductor and a transistor using the samewill be described in detail below.

As an example of the highly-purified oxide semiconductor, there is anoxide semiconductor whose carrier concentration is less than 1×10¹⁴/cm³, preferably less than 1×10¹²/cm³, and more preferably less than1×10¹¹/cm³ or less than 6.0×10¹⁰/cm³.

A transistor using a highly-purified oxide semiconductor ischaracterized in that the off-current is much lower than that of atransistor including a semiconductor containing silicon, for example.

The following shows the result of measuring the off-currentcharacteristics of a transistor with an evaluation element (alsoreferred to as TEG: Test Element Group). Note that the description ismade here on an n-channel transistor.

In the TEG, a transistor with L/W=3 μm/10000 μm, which includes 200transistors with L/W=3 μm/50 μm (thickness d:30 nm) connected inparallel, is provided. FIG. 28 illustrates the initial characteristicsof the transistor. Here, V_(G) is in the range of −20 V to +5 Vinclusive. In order to measure the initial characteristics of thetransistor, the characteristics of changes in the source-drain current(hereinafter, referred to as a drain current or I_(D)), i.e.,V_(G)-I_(D) characteristics, were measured under the conditions wherethe substrate temperature was set to room temperature, the voltagebetween the source and the drain (hereinafter, referred to as a drainvoltage or V_(D)) was set to 10 V, and the voltage between the sourceand the gate (hereinafter, referred to as a gate voltage or V_(G)) waschanged from −20 V to +20 V.

As illustrated in FIG. 28, the transistor with a channel width W of10000 μm has an off-current of 1×10⁻¹³ A or less at V_(D) of 1 V and 10V, which is less than or equal to the resolution (100 fA) of ameasurement device (a semiconductor parameter analyzer, Agilent 4156Cmanufactured by Agilent Technologies Inc.). The off-current permicrometer of the channel width corresponds to 10 aA/μm.

Note that in this specification, the off-current (also referred to asleakage current) means a current flowing between a source and a drain ofan n-channel transistor when a predetermined gate voltage in the rangeof −20 V to −5 V inclusive is applied at room temperature in the casewhere the n-channel transistor has a positive threshold voltage V_(th).Note that the room temperature is 15° C. to 25° C. inclusive. Atransistor including an oxide semiconductor that is disclosed in thisspecification has a current per unit channel width (W) of 100 aA/μm orless, preferably 1 aA/μm or less, and more preferably 10 zA/μm or lessat room temperature.

Moreover, the transistor including a high-purity oxide semiconductor hasfavorable temperature characteristics. Typically, in the temperaturerange of −25° C. to 150° C. inclusive, the current-voltagecharacteristics of the transistor, such as on-current, off-current,field-effect mobility, S value, and threshold voltage, hardly change anddeteriorate due to temperature. In addition, a high-purity oxidesemiconductor hardly deteriorates due to light irradiation, whichresults in an increase in the reliability of, in particular, asemiconductor device using light such as a photosensor.

Although the bottom-gate transistor 1004 is shown as an example in thisembodiment, a top-gate transistor 2004 may be used as illustrated inFIG. 20. The transistor 2004 includes a semiconductor film 2012, anelectrode 2013, an electrode 2014, a gate insulating film 2011, and agate electrode 2010.

Note that for each of the aforementioned semiconductor elements, a thinfilm semiconductor or a bulk semiconductor can be used.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 4)

This embodiment shows an example of the circuit, layout, andcross-sectional structure of the semiconductor device, which isdifferent from the example shown in Embodiments 1 to 3.

FIG. 21 is an example of the circuit diagram of the photosensor, inwhich the transistor 101 and the transistor 102 of FIG. 1 are omitted.

FIG. 22 is an example of the layout of the circuit in FIG. 21. Thelight-receiving area can be increased because of a reduction in thenumber of elements.

Since the transistor 103 and the transistor 104 are formed to overlapwith the n-region 201 of the PIN photodiode 100 in FIG. 22, thelight-receiving area can be increased. The light-receiving area can alsobe increased when the wiring 109 or the like is made of alight-transmitting material and is formed to overlap with the i-region202.

FIG. 23 is an example of the cross-sectional view of the photosensor inFIG. 21. A transistor 3001 corresponds to the transistor 103 in FIG. 21,and is formed to overlap with the n-region 1023 of the photodiode 1002.The transistor 3001, which is a bottom-gate thin film transistor,includes a gate electrode 3010, a gate insulating film 3011, asemiconductor film 3012, an electrode 3013, and an electrode 3014. Thetransistor 3001 may be a top-gate transistor like the transistor 2004illustrated in FIG. 20.

For the semiconductor film 3012, an amorphous semiconductor, amicrocrystal semiconductor, a polycrystalline semiconductor, an oxidesemiconductor, a single crystal semiconductor, or the like can be used.In particular, an electric signal can be amplified with high accuracy byusing an oxide semiconductor to obtain a transistor with an extremelylow off-current.

It is preferable that the transistor 104 in FIG. 21 be also formed overthe same surface and in the same process as the transistor 103 so as tooverlap with the n-region 1023. When an oxide semiconductor is used forthe transistor 104, an output signal can be read with high accuracy.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 5)

This embodiment shows an example of the circuit of the semiconductordevice, which is different from the example shown in the aboveembodiments.

FIGS. 27A to 27F are examples of the circuit of the photosensor. Thelayout or structure disclosed in this specification, which leads to anincrease in receiving area, can be applied to these circuits.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 6)

This embodiment shows an example of the cross-sectional structure of thesemiconductor device, which is different from the example shown inEmbodiment 3.

FIG. 24 is an example of the cross-sectional view of the photosensor.

In FIG. 24, a transistor 4001 is formed over the substrate 1001 havingan insulating surface, and a photodiode 4002 is formed over thetransistor 4001.

The transistor 4001 can be applied to all the transistors illustrated inFIG. 1, FIG. 7, FIG. 13, FIG. 21, and FIGS. 27A to 27F. That is, all thetransistors can be formed of the same material, over the same surface,and in the same process, resulting in cost reduction.

The photodiode 4002 is a vertical-junction PIN photodiode, in which ann-region 4003, an i-region 4004, and a p-region 4005 are stacked. Notethat the order of stacking can be a p-region, an i-region, and ann-region. The photodiode can be a PN photodiode without the i-region4004. For the photodiode 4002, an amorphous semiconductor, amicrocrystal semiconductor, a polycrystalline semiconductor, an oxidesemiconductor, a single crystal semiconductor, or the like can be used.In particular, silicon semiconductor is preferably used because thequantum efficiency of the photodiode 4002 can be improved.

The light 1202 emitted from the object to be detected 1201 enters thephotodiode 4002. With such a structure, there is no element blocking thelight 1202 entering the photodiode 4002; thus, a light-receiving areacan be made as large as possible.

For the transistor 4001, an amorphous semiconductor, a microcrystalsemiconductor, a polycrystalline semiconductor, an oxide semiconductor,a single crystal semiconductor, or the like can be used. In particular,light can be detected with high accuracy by using an oxide semiconductorto obtain a transistor with an extremely low off-current.

Note that some of the transistors included in the photosensor can beformed over a surface different from that over which the transistor 4001is formed. The other structure is the same as that shown in Embodiment3.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 7)

In this embodiment, an example of a semiconductor device using thephotosensor will be described.

FIG. 25 is an example of an image sensor provided with the photosensor.The image sensor includes a photosensor portion 5001 and a photosensorcontrol circuit 5002. The photosensor portion 5001 includes a pluralityof photosensors 5003 arranged in matrix. The photosensor control circuit5002 includes a photosensor reading circuit 5004 and a photosensordriver circuit 5005. The area sensor is shown here, though a line sensorcan also be used. The image sensor is applied to a digital still camera,a cellular phone, and the like.

The photosensor shown in the other embodiments can be applied to thephotosensors 5003.

FIG. 26 is an example of a display device provided with the photosensor.A display panel 6000 includes a pixel portion 6001, a display elementcontrol circuit 6002, and a photosensor control circuit 6003. The pixelportion 6001 includes pixels 6004 arranged in matrix, each of whichincludes a display element 6005 and a photosensor 6006. The displayelement control circuit 6002 includes display element driver circuits6007 and 6008. The photosensor control circuit 6003 includes aphotosensor reading circuit 6009 and a photosensor driver circuit 6010.The display device is applied to a touch panel and the like.

The photosensor shown in the other embodiments can be applied to thephotosensor 6006.

Note that the photosensor 6006 can be provided outside the pixel 6004.

As the display element 6005, a liquid crystal element, an EL element, anelectrophoretic element, or the like can be used.

The display element control circuit 6002 and the photosensor controlcircuit 6003 can be provided outside the display panel 6000.

This embodiment can be implemented in appropriate combination with theother embodiments.

(Embodiment 8)

In this embodiment, the operation of the semiconductor device will bedescribed.

The operation of the circuit in FIG. 1 will be described as an exampleof the operation of the photosensor. FIG. 29 is an example of a timingchart of the circuit in FIG. 1.

In FIG. 29, signals 301 to 305 respectively show the potentials of thewiring 107, the wiring 106, the wiring 109, the wiring 105, and thewiring 110 in FIG. 1. Here, the wiring 120 is assumed to be at theground potential.

At time A, the signal 301 is set to “H (High)” and the signal 302 is setto “H” (reset operation starts), and then, the photodiode 100 is turnedon and the signal 304 is set to “H”.

At time B, the signal 301 is set to “L (Low)” and the signal 302 is keptat “H” (the reset operation is completed and accumulation operationstarts). Then, the signal 304 starts to be lowered due to theoff-current of the photodiode 100. Since the off-current of thephotodiode 100 increases when light enters, the signal 304 changesdepending on the amount of incident light.

At time C, the signal 302 is set to “L” (the accumulation operation iscompleted), whereby the signal 304 becomes constant. Here, the signal304 is determined by the charge that has been supplied to the wiring 105from the photodiode 100 during the accumulation operation. That is, thecharge accumulated in the gate of the transistor 103 changes dependingon the light entering the photodiode 100.

At time D, the signal 303 is set to “H” (selection operation starts).Then, the transistor 104 is turned on, and electrical conduction isestablished between the wiring 108 and the wiring 110 through thetransistor 103 and the transistor 104. Thus, the signal 305 starts to belowered. The rate at which the signal 305 is lowered depends on thecurrent between the source and the drain of the transistor 103, namely,the amount of light that has emitted to the photodiode 100 during theaccumulation operation.

At time E, the signal 303 is set to “L” (the selection operation iscompleted), whereby the transistor 104 is turned off and the signal 305becomes constant. The constant value of the signal 305 changes dependingon the amount of light emitted to the photodiode 100. Consequently, theamount of light entering the photodiode 100 during the accumulationoperation can be found by obtaining the potential of the signal 305.

This embodiment can be implemented in appropriate combination with theother embodiments.

This application is based on Japanese Patent Application serial no.2010-034173 filed with Japan Patent Office on Feb. 19, 2010, the entirecontents of which are hereby incorporated by reference.

The invention claimed is:
 1. A semiconductor device comprising: alight-receiving element configured to convert light into an electricsignal; an insulating film over the light-receiving element; a firsttransistor configured to transfer the electric signal, one of a sourceand a drain of the first transistor being electrically connected to anoutput terminal of the light-receiving element; a second transistorconfigured to amplify the transferred electric signal, a gate of thesecond transistor being electrically connected to the other of thesource and the drain of the first transistor; and a third transistorconfigured to control reading of an output signal generated by thesecond transistor, one of a source and a drain of the third transistorbeing electrically connected to one of a source and a drain of thesecond transistor, wherein the first transistor comprises an oxidesemiconductor layer comprising the source and the drain, wherein theentire oxide semiconductor layer is provided over the insulating filmand overlaps with the insulating film, wherein a gate of the thirdtransistor extends over the light-receiving element, and wherein thefirst transistor is provided over the second transistor with theinsulating film provided therebetween.
 2. A semiconductor deviceaccording to claim 1, wherein the first transistor has an off-current of10 aA/μm or less.
 3. A semiconductor device according to claim 1,wherein the light-receiving element and a channel formation region ofthe second transistor are made from a same semiconductor layer.
 4. Asemiconductor device according to claim 1, wherein the light-receivingelement and the second transistor comprise silicon.
 5. A semiconductordevice according to claim 1, wherein the light-receiving element, achannel formation region of the second transistor, and a channelformation region of the third transistor are made from a samesemiconductor layer.
 6. A semiconductor device according to claim 1,further comprising a fourth transistor configured to control reset of agate potential of the second transistor; wherein a channel formationregion of the fourth transistor is made from a same layer as a channelformation region of the first transistor.
 7. A semiconductor deviceaccording to claim 1, further comprising a wiring provided over alight-receiving portion of the light-receiving element, wherein thewiring comprises a light-transmitting material.
 8. A semiconductordevice according to claim 1, wherein the insulating film is providedover the light-receiving element and the second transistor.
 9. Asemiconductor device according to claim 1, further comprising a wiring,wherein the wiring comprises a light-transmitting material, is providedover the light-receiving element, and is electrically connected to thegate of the third transistor.
 10. A semiconductor device comprising: aphotodiode comprising a semiconductor film provided on a surface, thesemiconductor film comprising an n-region and a p-region each in contactwith the surface; an insulating film over the photodiode; a firsttransistor configured to transfer an electric signal generated by thephotodiode, one of a source and a drain of the first transistor beingelectrically connected to an output terminal of the photodiode; a secondtransistor configured to amplify the transferred electric signal, a gateof the second transistor being electrically connected to the other ofthe source and the drain of the first transistor; and a third transistorconfigured to control reading of an output signal generated by thesecond transistor, one of a source and a drain of the third transistorbeing electrically connected to one of a source and a drain of thesecond transistor, wherein the first transistor comprises an oxidesemiconductor layer comprising the source and the drain and overlapswith one of the n-region and the p-region of the photodiode when seenfrom above, wherein the entire oxide semiconductor layer is providedover the insulating film and overlaps with the insulating film, whereina gate of the third transistor extends over the p-region of thesemiconductor film, and wherein the first transistor is provided overthe second transistor with the insulating film provided therebetween.11. A semiconductor device according to claim 10, wherein the n-region,the p-region and a channel formation region of the second transistor aremade from the same semiconductor layer.
 12. A semiconductor deviceaccording to claim 10, wherein the photodiode and the second transistorcomprise silicon.
 13. A semiconductor device according to claim 10,wherein the n-region, the p-region, a channel formation region of thesecond transistor, and a channel formation region of the thirdtransistor are made from a same semiconductor layer.
 14. A semiconductordevice according to claim 10, further comprising a fourth transistorconfigured to control reset of a gate potential of the secondtransistor; wherein a channel formation region of the fourth transistoris made from a same layer as a channel formation region of the firsttransistor.
 15. A semiconductor device according to claim 10, furthercomprising a wiring provided over the photodiode, wherein the wiringcomprises a light-transmitting material.
 16. A semiconductor deviceaccording to claim 10, further comprising a wiring, wherein thephotodiode is a lateral junction PIN photodiode, wherein thesemiconductor film comprises an i-region in contact with the surface,and wherein the wiring comprises a light-transmitting material, isprovided over the i-region of the semiconductor film, and iselectrically connected to the gate of the third transistor.
 17. Asemiconductor device comprising: a light-receiving element configured toconvert light into an electric signal; an insulating film over thelight-receiving element; a first transistor, one of a source and a drainof the first transistor being electrically connected to an outputterminal of the light-receiving element; a second transistor, a gate ofthe second transistor being electrically connected to the other of thesource and the drain of the first transistor; and a third transistor,one of a source and a drain of the third transistor being electricallyconnected to one of a source and a drain of the second transistor,wherein the first transistor comprises an oxide semiconductor layercomprising the source and the drain, wherein the entire oxidesemiconductor layer is provided over the insulating film and overlapswith the insulating film, wherein a gate of the third transistor extendsover the light-receiving element, and wherein the first transistor isprovided over the second transistor with the insulating film providedtherebetween.
 18. A semiconductor device according to claim 17, whereinthe first transistor has an off-current of 10 aA/μm or less.
 19. Asemiconductor device according to claim 17, wherein the light-receivingelement and a channel formation region of the second transistor are madefrom a same semiconductor layer.
 20. A semiconductor device according toclaim 17, wherein the light-receiving element and the second transistorcomprise silicon.
 21. A semiconductor device according to claim 17,wherein the light-receiving element, a channel formation region of thesecond transistor, and a channel formation region of the thirdtransistor are made from a same semiconductor layer.
 22. A semiconductordevice according to claim 17, further comprising a fourth transistor,one of a source and a drain of the fourth transistor being electricallyconnected to the gate of the second transistor; wherein a channelformation region of the fourth transistor is made from a same layer as achannel formation region of the first transistor.
 23. A semiconductordevice according to claim 17, further comprising a wiring provided overa light-receiving portion of the light-receiving element, wherein thewiring comprises a light-transmitting material.
 24. A semiconductordevice according to claim 17, further comprising a wiring, wherein thewiring comprises a light-transmitting material, is provided over thelight-receiving element, and is electrically connected to the gate ofthe third transistor.
 25. A semiconductor device comprising: aphotodiode comprising a semiconductor film provided on a surface, thesemiconductor film comprising an n-region and a p-region each in contactwith the surface; an insulating film over the photodiode; a firsttransistor, one of a source and a drain of the first transistor beingelectrically connected to an output terminal of the photodiode; a secondtransistor, a gate of the second transistor being electrically connectedto the other of the source and the drain of the first transistor; and athird transistor, one of a source and a drain of the third transistorbeing electrically connected to one of a source and a drain of thesecond transistor, wherein the first transistor comprises an oxidesemiconductor layer comprising the source and the drain and overlapswith one of the n-region and the p-region of the photodiode when seenfrom above, wherein the entire oxide semiconductor layer is providedover the insulating film and overlaps with the insulating film, whereina gate of the third transistor extends over the p-region of thesemiconductor film, and wherein the first transistor is provided overthe second transistor with the insulating film provided therebetween.26. A semiconductor device according to claim 25, wherein the n-region,the p-region and a channel formation region of the second transistor aremade from the same semiconductor layer.
 27. A semiconductor deviceaccording to claim 25, wherein the photodiode and the second transistorcomprise silicon.
 28. A semiconductor device according to claim 25,wherein the n-region, the p-region, a channel formation region of thesecond transistor, and a channel formation region of the thirdtransistor are made from a same semiconductor layer.
 29. A semiconductordevice according to claim 25, further comprising a fourth transistor,one of a source and a drain of the fourth transistor being electricallyconnected to the gate of the second transistor; wherein a channelformation region of the fourth transistor is made from a same layer as achannel formation region of the first transistor.
 30. A semiconductordevice according to claim 25, further comprising a wiring provided overa light-receiving portion of the photodiode, wherein the wiringcomprises a light-transmitting material.